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seen to be much more definite than that expressed in the term combining weight. The former term brings before the mind the picture of a small definite portion of matter with definite properties; the latter term merely expresses a ratio. The values of atomic weights are determined by two methods, of general applicability, which are deduced from the principles of a theory of the structure of matter which gives a fairly simple explanation of most, if not all, of the observed physical properties of matter. The value given to the combining weight of an element is a purely empirical value; it must be determined for each element by methods specially applicable to that element; it is one of several possible values, and it is selected on the ground of general convenience and expediency.
The conception expressed in the term molecular weight is 320 also much more definite than that underlying the expression reacting weight. The molecule of a gas is a perfectly definite quantity of matter with defined properties; it is a physical conception, deduced by dynamical reasoning from a physical theory of the structure of matter. This theory presents us with one generally applicable method for determining the relative weights of gaseous molecules. The reacting weight of a body is said to be the smallest relative mass of it which takes part in chemical interactions’; but this statement involves terms which cannot, at present at any rate, be accurately defined. The value which, under the circumstances, is the best to be given to the reacting weight of a substance must be deduced for each substance by methods which to a very great extent are empirical, and many of which are applicable only to the special case under consideration. It is true that the molecular and atomic theory has not yet enabled us to define the term molecule as applied to a liquid or solid body; but the theory has thrown light on the chemical conception of reacting weight, and in place of regarding the values of reacting weights merely as numbers, we may now look on them as expressing the relative weights of certain aggregations of atoms which interact with each other to produce new aggregations of atoms.
The molecular and atomic theory then regards the properties 321 of a gas as the properties of the molecules of that gas; and the properties of the molecules as dependent, among other conditions, on the nature and number of the atoms which form these molecules. But if this view is correct, and if it is true that the number of atoms in the molecule of a gaseous element
vary, we should expect sometimes to find differences in the properties of one and the same element.
The descriptions given in Chap. XI. pars. 173, 220, 221, of the properties of sulphur and phosphorus shewed that each of these elements differs very considerably in properties under different conditions. But as both elements exhibit differences in the solid state we cannot say whether these differences are, or are not, connected with differences in the atomicities of the molecules of the elements.
Some of the prominent properties of oxygen were described in Chap. VIII. pars. 118 to 121. If a quantity of pure dry oxygen is confined over sulphuric acid and a series of electric induction-sparks is passed through the oxygen, the volume of the gas diminishes until it has become about 1 less than it was at the beginning of the experiment. The properties of the gas after the diminution of volume has ceased are markedly different from those of oxygen; nevertheless it has been conclusively proved that nothing has combined with the oxygen during the change which has occurred. The new gas is called ozone (because of its smell). The whole of a specified quantity of oxygen cannot be changed into ozone, so that pure ozone has not yet been obtained. But experiments have proved ; (1) that the relation between the volume of that portion of a quantity of oxygen which is changed into ozone, and the volume of the ozone formed is expressed by the ratio 3 : 2; and (2) that the weight of the ozone produced is equal to the weight of the oxygen which has been changed into ozone. If the gas through which electric sparks have been sent until diminution of volume has ceased is heated to 360° or so, expansion occurs, and the original volume of oxygen is reproduced.
The outcome of the experiments on the volume- and massrelations between oxygen and ozone is that the change of oxygen to ozone, or vice versa, is attended with no change of mass, but that 3 volumes of oxygen condense to 2 vols. of ozone, and 2 volumes of ozone are changed by heat to 3 volumes of oxygen. Now as oxygen is 16 times heavier than hydrogen, it follows that ozone is 24 times heavier than hydrogen ; therefore, as the molecular weight of a gas is twice its specific gravity referred to hydrogen, it follows that the molecular weight of ozone is 24 x 2 = 48. As ozone is only modified oxygen, and as the atomic weight of oxygen is 16, the molecule of ozone must be composed of 3 atoms of oxygen. Therefore the atomic composition of the molecule of ozone is expressed by the symbol 03, that of the molecule of oxygen being expressed by the symbol 0.
Oxygen and ozone are both colourless gases ; oxygen is odourless, ozone has a very pronounced odour; ozone is a very energetic oxidiser, e. g. when passed into mercury at ordinary temperatures it produces mercuric oxide, and it interacts with lead sulphide (PbS) to produce lead sulphate (PbSO.); oxygen does not react with an aqueous solution of potassium iodide (KI), ozone interacts with this salt in aqueous solution and produces potassium oxide, iodine, and oxygen; thus
2KIAq+=KOAq+1, +0 The existence of the two kinds of molecules, 0, and Og, 324 one diatomic and the other triatomic, each characterised by its own properties yet each composed of the same kind of atoms, and of atoms all of which are alike, shews that the properties of some molecules at any rate are conditioned (among other circumstances) by the number of atoms which are combined to form these molecules.
The other cases of allotropy (s. Chap. XI. par. 173) exhibited 325 by elements are exhibited by those elements in the solid state. The numbers of atoms in the atomic aggregates which compose the reacting weights of the different solid forms of phosphorus, sulphur, arsenic, &c. may perhaps be different; but we cannot at present decide whether this is so or not.
If the properties of a gas are dependent only on the 326 nature and number of the atoms which form the molecule of that gas, then the existence of more than one gaseous compound having a specified composition must be impossible. But, as a matter of fact, several compounds frequently exist, all having the same composition and the same molecular weight. For instance 3 compounds having the composition CH, are known; these bodies have all been gasified; their specific gravities as gases, and therefore their molecular weights, are identical. But the molecule of each compound is composed of 5 atoms of carbon united with 12 atoms of hydrogen. Therefore we conclude that the properties of some gaseous molecules are conditioned by other circumstances besides the nature and number of their constituent atoms, and that one of these other circumstances probably is the arrangement of the parts of the molecule relatively to each other.
The existence of more than one compound with the same
molecular composition is called isomerism. We shall return to this subject in the next chapter.
We now see how necessary it has become to widen our conception of chemical composition. Restricting ourselves to gases, we have found that there may exist more than one form of the same element—we have found for instance that there are two oxygens; we have also learnt that the same quantities of the same elements may be combined so as to produce different compounds; and that the same numbers of the same atoms may be combined so as to produce several kinds of molecules, each differing in properties from the others.
The properties of an element or compound are the properties of its molecules; the properties of these molecules are conditioned not only by the nature, but also by the number, of atoms which form them; but the properties of these molecules, it appears, are also probably conditioned by the relative arrangement of their parts.
This conception of the relation between chemical properties and composition helps us to understand, more fully than we could do before, the relations between the composition and the properties of such classes of compounds as acids, alkalis, and salts.
In Chaps. VIII. and ix. we learned that the compounds of hydrogen with oxygen and another negative element, or other negative elements, are generally acids, and that the compounds of hydrogen with oxygen and a markedly positive element are alkalis. We also found that the lower oxides of elements which are neither very markedly positive or negative are usually basic, but that the combination of more oxygen with such oxides produces acidic oxides.
We may now translate these statements into the language of the molecular and atomic theory, and say that molecules formed by the union of atoms of hydrogen and oxygen with atoms of negative elements interact, under proper conditions, with molecules of metals, basic oxides, or alkalis, and exchange some or all of their atoms of hydrogen for atoms of metal. We may also say that molecules composed of fairly positive atoms united with a small number of atoms of
oxygen are ready to exchange their positive atoms for atoms of hydrogen when they interact with acids, under suitable conditions; but that molecules composed of many oxygen atoms united with a small number of atoms of fairly positive elements do not thus exchange their positive atoms for hydrogen, but on the other hand are ready to interact with water to produce acids.
As few acids, alkalis, basic oxides, or salts, have been gasified, the term molecule is used in the foregoing paragraph to include the aggregates of atoms which form the reacting weights of solid bodies.
We know that acids may be classified in accordance with 329 their basicity (s. Chap. XI. pars. 188 and 189). Instead of the statement that 'an n-basic acid is an acid from the reacting weight of which n combining weights of hydrogen can be displaced by sodium or potassium under suitable conditions'; we may now say that the molecule of an n-basic acid contains n atoms of replaceable hydrogen. It must be carefully noted that the term molecule is here used with a wider meaning than theory strictly justifies; it must be taken to mean that aggregate of atoms (this may or may not be a true molecule) which forms the reacting weight of an acid, &c. The account of the reactions of acids given in Chaps. x. and xi. shews what is meant by replaceable hydrogen. The compositions of a few acids and of some of the salts derived from them are presented in the following table ; by considering the relations between the compositions of these acids and their salts, the student will be better able to grasp the meaning of the definition of the basicity of an acid which has been given. The formulae represent reacting atomic aggregates, which in some cases are probably true molecules. Acids.
Fe (H.C,0), &c. . In Chap. XII. we learned a little about the relative affinities 330 of acids. A comparison of the relative affinities of a series of acids the compositions of which differ only by the relative quantities of oxygen the acids contain brings out the fact, that the affinity-constants of the acids generally increase as the quantity of oxygen increases; thus the relative affinities of the three acids H,SO3, H, SO4, H,S,Oo, are in the ratio 66 : 150 : 178. Again the replacement of hydrogen by a strongly negative